To adjust the text, you could modify the login_required phrase. Alternatively, you could look at adding a notice for guests.

To allow a single forum to be visible, you need to ensure guests have the general view permission. You'll then need to remove access to all the other nodes individually (revoke the view node permission from them).

Can I trouble you to tell me how to modify the login_required phrase? I don't see that in the options and with the underbar it sounds a little more behind the scenes (which is fine, I don't mind getting my hands dirty.)

Modifying a phrase is straightforward and roughly equivalent to changing a template. In the ACP, Appearance > Phrases > use the filter for "login_required" > click the result > edit the "phrase text" (not the title) to suit your needs. (I believe you should be able to use HTML here, though this isn't always true.)